Molecular mechanism for rapid autoxidation in α-pinene ozonolysis

Key route for atmospheric organic aerosol formation is experimentally confirmed by utilizing MION’s capabilities.


α-pinene is the most emitted monoterpene and a very efficient precursor of secondary organic aerosol (SOA), with reaction (α-pinene + ozone (O3)) being one of the main SOA-forming systems in the atmosphere. However, the known pathways of the reaction could not fully explain the rapid SOA formation. 

Recently, some VOC have been shown to rapidly undergo autoxidation, a series of sequential peroxy radical (RO2) hydrogen shifts (H-shifts) and O2 additions to the hydrocarbon chain, leading to highly oxygenated organic molecules (HOM). In the previously proposed mechanisms, the steric strain of the cyclobutyl ring hindered the H-shifts in the RO2 isomers.


Quantum chemical calculations of the route that the reaction (α-pinene +O3) can take followed by a targeted laboratory experiment to confirm the sub-second timescales of the reaction. Laboratory experiments were performed with a Multi-scheme chemical IONization inlet (MION) coupled to the time-of-flight mass spectrometer and using nitrate as a reagent ion source.

Highlighted results:

A new pathway for the α-pinene ozonolysis reaction is proposed that shows a possibility of forming RO2 without steric strain using the excess energy from the initial reaction. The proposed autoxidation steps include (Fig. 1): a formation of a six-member endoperoxide from the RO2 via the transition state TS-endo, which subsequently adds an O2 to form a six-oxygen containing peroxy radical O6-RO2. This molecule can undergo a fast 1,4 aldehydic H-shift to form O6-hydroperoxide via the transition state TS-1,4 H-shift, which subsequently adds an O2 to form an eight-oxygen containing peroxy radical O8-RO2.

Figure 1: Autoxidation steps available to ring-broken RB1-RO2.

The speed of the peroxy radical C10H15O8 formation was explored in the laboratory settings for precursor residence times between 75 and 1000 ms with the inlet flow rates 20 or 30 l per minute. The experiments confirmed the extremely rapid formation of C10H15O8 – within 75-ms residence time as shown in the Figure 2. The next ten-oxygen containing RO2 was formed within 300 ms. 

Figure 2: NO3-MION2-MS spectra for α-pinene ozonolysis. The background peak at 75 ms in a is expanded in c. Blue labels indicate radical and green labels indicate closed-shell species derived from α-pinene ozonolysis. Red label indicates possible radical species from OH oxidation of α-pinene.

Reference: Iyer, S., Rissanen, M.P., Valiev, R. et al. Molecular mechanism for rapid autoxidation in α-pinene ozonolysis. Nat Commun 12, 878 (2021). 

The article is licensed under a Creative Commons Attribution 4.0 International License, to view a copy of this license, visit

Download this summary:

Mechanism for rapid autoxidation

More resources

The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source


Pesticide residue fast screening using Thermal Desorption MION MS with selective chemical ionization


Molecular composition and volatility of gaseous organic compounds in a boreal forest

View all resources